Detection system for power equipment

ABSTRACT

A machine is disclosed having a detection subsystem adapted to detect contact between a person and a specified portion of the machine, and to distinguish that contact from contact with other materials. The detection subsystem imparts an electrical signal to a specified portion of the machine, and distinguishes that contact based on a predetermined frequency response of the electrical signal. A reaction subsystem then causes a predetermined action to take place. The machine may be a power saw designed to minimize injury in the event a person accidentally contacts the blade.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority from the followingU.S. Provisional Patent Application, the disclosure of which is hereinincorporated by reference: Ser. No. 60/335,970, filed Nov. 13, 2001.

TECHNICAL FIELD

The present invention relates to detecting contact between a body part,and a sensor and distinguishing such contact from contact between thesensor and other objects.

BACKGROUND

There are many circumstances where it is beneficial to be able todistinguish contact with a human body from contact with other objects ormaterials. One area where such a capability is especially important isin the guarding of dangerous power equipment. For instance, a systemadapted to detect accidental contact between the user of a saw and thesaw blade is described in U.S. Provisional Patent Application Ser. No.60/225,200, filed Aug. 14, 2000 and U.S. patent application Ser. No.09/929,426, filed Aug. 13, 2001, which are incorporated herein byreference and are assigned to the assignee of the present application.The system of that application relies on the inherent capacitance of thehuman body to change the voltage on a saw blade carrying a highfrequency signal. The system monitors the voltage on the blade, and whenit drops suddenly due to contact with a body, the system signals a highspeed brake to stop the blade.

While the above-incorporated applications describe variousconfigurations and features which allow the system to distinguishvoltage drops caused by contact between the blade and a person fromvoltage drops caused by other events, additional configurations arepossible as describe below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a machine with a fast-actingsafety system according to the present invention.

FIG. 2 is a schematic diagram of an exemplary safety system in thecontext of a machine having a circular blade.

FIG. 3 is an equivalent circuit model that generally characterizes theelectrical system formed by an exemplary detection subsystem and bladewhen the blade is in contact with a human body.

FIG. 4 is a graph schematically illustrating exemplary drive and sensedsignals according to the present invention.

FIG. 5 is similar to FIG. 4 but shows the sensed signal reduced inamplitude due to increase apparent capacitance of the blade.

FIG. 6 is a graph schematically illustrating an alternative drive signalhaving a period substantially equivalent to the RC-time constant of ahuman body, and showing the corresponding sensed signal when the humanbody is coupled to the blade.

FIG. 7 is a graph schematically illustrating an alternative drive signalhaving a period equal to approximately five RC-time constants of a humanbody, and showing the corresponding sensed signal when the human body iscoupled to the blade.

FIG. 8 is a graph schematically illustrating an alternative drive signalhaving a period equal to approximately 1/10^(th) of an RC-time constantof a human body, and showing the corresponding sensed signal when thehuman body is coupled to the blade.

FIG. 9 is a flowchart illustrating an exemplary method for configuring adetection subsystem to distinguish contact between a cutting tool and aperson from contact between a cutting tool and other materials.

FIG. 10 is a graph showing an exemplary frequency response graphcorresponding to the sensed signal from an exemplary cutting tool incontact with a person.

FIG. 11 is a graph showing an exemplary frequency response correspondingto the sensed signal from an exemplary cutting tool in contact with amaterial other than a person.

FIG. 12 is an equivalent circuit model that generally characterizes theelectrical system formed by a blade and an alternative exemplarydetection subsystem having a test electrode.

DETAILED DESCRIPTION

A machine according to the present invention is shown schematically inFIG. 1 and indicated generally at 10. Machine 10 may be any of a varietyof different machines adapted for cutting workpieces, such as wood,including a table saw, miter saw (chop saw), radial arm saw, circularsaw, band saw, jointer, planer, etc. Machine 10 includes an operativestructure 12 having a cutting tool 14 and a motor assembly 16 adapted todrive the cutting tool. Machine 10 also includes a safety system 18configured to minimize the potential of a serious injury to a personusing machine 10. Safety system 18 is adapted to detect the occurrenceof one or more dangerous conditions during use of machine 10. If such adangerous condition is detected, safety system 18 is adapted to engageoperative structure 12 to limit any injury to the user caused by thedangerous condition.

Machine 10 also includes a suitable power source 20 to provide power tooperative structure 12 and safety system 18. Power source 20 may be anexternal power source such as line current, or an internal power sourcesuch as a battery. Alternatively, power source 20 may include acombination of both external and internal power sources. Furthermore,power source 20 may include two or more separate power sources, eachadapted to power different portions of machine 10.

It will be appreciated that operative structure 12 may take any one ofmany different forms, depending on the type of machine 10. For example,operative structure 12 may include a stationary housing configured tosupport motor assembly 16 in driving engagement with cutting tool 14.Alternatively, operative structure 12 may include a movable structureconfigured to carry cutting tool 14 between multiple operatingpositions. As a further alternative, operative structure 12 may includeone or more transport mechanisms adapted to convey a workpiece towardand/or away from cutting tool 14.

Motor assembly 16 includes one or more motors adapted to drive cuttingtool 14. The motors may be either directly or indirectly coupled to thecutting tool, and may also be adapted to drive workpiece transportmechanisms. Cutting tool 14 typically includes one or more blades orother suitable cutting implements that are adapted to cut or removeportions from the workpieces. The particular form of cutting tool 14will vary depending upon the various embodiments of machine 10. Forexample, in table saws, miter saws, circular saws and radial arm saws,cutting tool 14 will typically include one or more circular rotatingblades having a plurality of teeth disposed along the perimetrical edgeof the blade. For a jointer or planer, the cutting tool typicallyincludes a plurality of radially spaced-apart blades. For a band saw,the cutting tool includes an elongate, circuitous tooth-edged band.

Safety system 18 includes a detection subsystem 22, a reaction subsystem24 and a control subsystem 26. Control subsystem 26 may be adapted toreceive inputs from a variety of sources including detection subsystem22, reaction subsystem 24, operative structure 12 and motor assembly 16.The control subsystem may also include one or more sensors adapted tomonitor selected parameters of machine 10. In addition, controlsubsystem 26 typically includes one or more instruments operable by auser to control the machine. The control subsystem is configured tocontrol machine 10 in response to the inputs it receives.

Detection subsystem 22 is configured to detect one or more dangerous, ortriggering, conditions during use of machine 10. For example, thedetection subsystem may be configured to detect that a portion of theuser's body is dangerously close to, or in contact with, a portion ofcutting tool 14. As another example, the detection subsystem may beconfigured to detect the rapid movement of a workpiece due to kickbackby the cutting tool, as is described in U.S. Provisional PatentApplication Ser. No. 60/182,866, filed Feb. 16, 2000 and U.S. patentapplication Ser. No. 09/676,190, filed Sep. 29, 2000, the disclosures ofwhich are herein incorporated by reference. In some embodiments,detection subsystem 22 may inform control subsystem 26 of the dangerouscondition, which then activates reaction subsystem 24. In otherembodiments, the detection subsystem may be adapted to activate thereaction subsystem directly.

Once activated in response to a dangerous condition, reaction subsystem24 is configured to engage operative structure 12 quickly to preventserious injury to the user. It will be appreciated that the particularaction to be taken by reaction subsystem 24 will vary depending on thetype of machine 10 and/or the dangerous condition that is detected. Forexample, reaction subsystem 24 may be configured to do one or more ofthe following: stop the movement of cutting tool 14, disconnect motorassembly 16 from power source 20, place a barrier between the cuttingtool and the user, or retract the cutting tool from its operatingposition, etc. The reaction subsystem may be configured to take acombination of steps to protect the user from serious injury. Placementof a barrier between the cutting tool and teeth is described in moredetail in U.S. Provisional Patent Application Ser. No. 60/225,206, filedAug. 14, 2000 and U.S. patent application Ser. No. 09/929,226, filedAug. 13, 2001, the disclosures of which are herein incorporated byreference. Retraction of the cutting tool from its operating position isdescribed in more detail in U.S. Provisional Patent Application Ser. No.60/225,089, filed Aug. 14, 2000 and U.S. patent application Ser. No.09/929,242, filed Aug. 13, 2001, the disclosures of which are hereinincorporated by reference.

The configuration of reaction subsystem 24 typically will vary dependingon which action(s) are taken. In the exemplary embodiment depicted inFIG. 1, reaction subsystem 24 is configured to stop the movement ofcutting tool 14 and includes a brake mechanism 28, a biasing mechanism30, a restraining mechanism 32, and a release mechanism 34. Brakemechanism 28 is adapted to engage operative structure 12 under theurging of biasing mechanism 30. During normal operation of machine 10,restraining mechanism 32 holds the brake mechanism out of engagementwith the operative structure. However, upon receipt of an activationsignal by reaction subsystem 24, the brake mechanism is released fromthe restraining mechanism by release mechanism 34, whereupon, the brakemechanism quickly engages at least a portion of the operative structureto bring the cutting tool to a stop.

It will be appreciated by those of skill in the art that the exemplaryembodiment depicted in FIG. 1 and described above may be implemented ina variety of ways depending on the type and configuration of operativestructure 12. Turning attention to FIG. 2, one example of the manypossible implementations of safety system 18 is shown. System 18 isconfigured to engage an operative structure having a cutting tool in theform of a circular blade 40 mounted on a rotating shaft or arbor 42.Blade 40 includes a plurality of cutting teeth (not shown) disposedaround the outer edge of the blade. As described in more detail below,brake mechanism 28 is adapted to engage the teeth of blade 40 and stopthe rotation of the blade. U.S. Provisional Patent Application Ser. No.60/225,210, filed Aug. 14, 2000 and U.S. patent application Ser. No.09/929,425, filed Aug. 13, 2001, the disclosures of which are hereinincorporated by reference, describe other systems for stopping themovement of the cutting tool. U.S. Provisional Patent Application Ser.No. 60/225,057, filed Aug. 14, 2000 U.S. patent application Ser. No.09/929,238, filed Aug. 13, 2001, U.S. Provisional Patent ApplicationSer. No. 60/225,058, filed Aug. 14, 2000, and U.S. patent applicationSer. No. 09/929,235, filed Aug. 13, 2001, the disclosures of which areherein incorporated by reference, describe safety system 18 in thecontext of particular types of machines 10.

In the exemplary implementation, detection subsystem 22 is adapted todetect the dangerous condition of the user coming into contact withblade 40. The detection subsystem includes a sensor assembly, such ascontact detection plates 44 and 46, capacitively coupled to blade 40 todetect any contact between the user's body and the blade. Typically, theblade, or some larger portion of cutting tool 14, is electricallyisolated from the remainder of machine 10. Alternatively, detectionsubsystem 22 may include a different sensor assembly configured todetect contact in other ways, such as optically, resistively, etc. Inany event, the detection subsystem is adapted to transmit a signal tocontrol subsystem 26 when contact between the user and the blade isdetected. Various exemplary embodiments and implementations of detectionsubsystem 22 are described in U.S. Provisional Patent Application Ser.No. 60/225,200, filed Aug. 14, 2000, U.S. patent application Ser. No.09/929,426, filed Aug. 13, 2001, U.S. Provisional Patent ApplicationSer. No. 60/225,211, filed Aug. 14, 2000, U.S. patent application Ser.No. 09/929,221, filed Aug. 13, 2001, U.S. Provisional Patent ApplicationSer. No. 60/270,011, filed Feb. 20, 2001, U.S. Provisional PatentApplication Ser. No. 60/298,207, filed Jun. 13, 2001, and U.S.Provisional Patent Application Ser. No. 60/302,937, filed Jul. 2, 2001,the disclosures of which are herein incorporated by reference.

Control subsystem 26 includes one or more instruments 48 that areoperable by a user to control the motion of blade 40. Instruments 48 mayinclude start/stop switches, speed controls, direction controls, etc.Control subsystem 26 also includes a logic controller 50 connected toreceive the user's inputs via instruments 48. Logic controller 50 isalso connected to receive a contact detection signal from detectionsubsystem 22. Further, the logic controller may be configured to receiveinputs from other sources (not shown) such as blade motion sensors,workpiece sensors, etc. In any event, the logic controller is configuredto control operative structure 12 in response to the user's inputsthrough instruments 48. However, upon receipt of a contact detectionsignal from detection subsystem 22, the logic controller overrides thecontrol inputs from the user and activates reaction subsystem 24 to stopthe motion of the blade. Various exemplary embodiments andimplementations of control subsystem 26 are described in more detail inU.S. Provisional Patent Application Ser. No. 60/225,059, filed Aug. 14,2000, U.S. patent application Ser. No. 09/929,237, filed Aug. 13, 2001,U.S. Provisional Patent Application Ser. No. 60/225,094, filed Aug. 14,2000, and U.S. patent application Ser. No. 09/929,234, filed Aug. 13,2001, the disclosures of which are herein incorporated by reference.

In the exemplary implementation, brake mechanism 28 includes a pawl 60mounted adjacent the edge of blade 40 and selectively moveable to engageand grip the teeth of the blade. Pawl 60 may be constructed of anysuitable material adapted to engage and stop the blade. As one example,the pawl may be constructed of a relatively high strength thermoplasticmaterial such as polycarbonate, ultrahigh molecular weight polyethylene(UHMW) or Acrylonitrile Butadiene Styrene (ABS), etc., or a metal suchas aluminum, etc. It will be appreciated that the construction of pawl60 will vary depending on the configuration of blade 40. In any event,the pawl is urged into the blade by a biasing mechanism in the form of aspring 66. In the illustrative embodiment shown in FIG. 2, pawl 60 ispivoted into the teeth of blade 40. It should be understood that slidingor rotary movement of pawl 60 might also be used. The spring is adaptedto urge pawl 60 into the teeth of the blade with sufficient force togrip the blade and quickly bring it to a stop.

The pawl is held away from the edge of the blade by a restrainingmechanism in the form of a fusible member 70. The fusible member isconstructed of a suitable material adapted to restrain the pawl againstthe bias of spring 66, and also adapted to melt under a determinedelectrical current density. Examples of suitable materials for fusiblemember 70 include NiChrome wire, stainless steel wire, etc. The fusiblemember is connected between the pawl and a contact mount 72. Preferably,fusible member 70 holds the pawl relatively close to the edge of theblade to reduce the distance the pawl must travel to engage the blade.Positioning the pawl relatively close to the edge of the blade reducesthe time required for the pawl to engage and stop the blade. Typically,the pawl is held approximately 1/32-inch to ¼-inch from the edge of theblade by fusible member 70, however other pawl-to-blade spacings mayalso be used within the scope of the invention.

Pawl 60 is released from its unactuated, or cocked, position to engageblade 40 by a release mechanism in the form of a firing subsystem 76.The firing subsystem is coupled to contact mount 72, and is configuredto melt fusible member 70 by passing a surge of electrical currentthrough the fusible member. Firing subsystem 76 is coupled to logiccontroller 50 and activated by a signal from the logic controller. Whenthe logic controller receives a contact detection signal from detectionsubsystem 22, the logic controller sends an activation signal to firingsubsystem 76, which melts fusible member 70, thereby releasing the pawlto stop the blade. Various exemplary embodiments and implementations ofreaction subsystem 24 are described in more detail in U.S. ProvisionalPatent Application Ser. No. 60/225,056, filed Aug. 14, 2000, U.S. patentapplication Ser. No. 09/929,240, filed Aug. 13, 2001, U.S. ProvisionalPatent Application Ser. No. 60/225,169, filed Aug. 14, 2000, U.S. patentapplication Ser. No. 09/929,241, filed Aug. 13, 2001, U.S. ProvisionalPatent Application Ser. No. 60/225,170, filed Aug. 14, 2000, and U.S.patent application Ser. No. 09/929,227, filed Aug. 13, 2001, thedisclosures of which are herein incorporated by reference.

It will be appreciated that activation of the brake mechanism willrequire the replacement of one or more portions of safety system 18. Forexample, pawl 60 and fusible member 70 typically must be replaced beforethe safety system is ready to be used again. Thus, it may be desirableto construct one or more portions of safety system 18 in a cartridgethat can be easily replaced. For example, in the exemplaryimplementation depicted in FIG. 2, safety system 18 includes areplaceable cartridge 80 having a housing 82. Pawl 60, spring 66,fusible member 70 and contact mount 72 are all mounted within housing82. Alternatively, other portions of safety system 18 may be mountedwithin the housing. In any event, after the reaction system has beenactivated, the safety system can be reset by replacing cartridge 80. Theportions of safety system 18 not mounted within the cartridge may bereplaced separately or reused as appropriate. Various exemplaryembodiments and implementations of a safety system using a replaceablecartridge are described in more detail in U.S. Provisional PatentApplication Ser. No. 60/225,201, filed Aug. 14, 2000, U.S. patentapplication Ser. No. 09/929,236, filed Aug. 13, 2001, U.S. ProvisionalPatent Application Ser. No. 60/225,212, filed Aug. 14, 2000, and U.S.patent application Ser. No. 09/929,244, filed Aug. 13, 2001, thedisclosures of which are herein incorporated by reference.

While one particular implementation of safety system 18 has beendescribed, it will be appreciated that many variations and modificationsare possible within the scope of the invention. Many such variations andmodifications are described in U.S. Provisional Patent Application Ser.No. 60/157,340, filed Oct. 1, 1999, U.S. Provisional Patent ApplicationSer. No. 60/182,866, filed Feb. 16, 2000, and U.S. patent applicationSer. No. 09/676,190, filed Sep. 29, 2000, the disclosures of which areherein incorporated by reference.

Considering detection subsystem 22 in more detail, the referencesincorporated above describe a variety of different exemplary detectionsubsystems adapted to detect contact between a person and blade 40. Forexample, several detection subsystems described in U.S. ProvisionalPatent Application Ser. No. 60/225,200, filed Aug. 14, 2000, and U.S.patent application Ser. No. 09/929,426, filed Aug. 13, 2001 areconfigured to detect any change in the apparent electrical capacitanceof the blade. FIG. 3 schematically illustrates the basic electricalcircuit equivalent of a typical detection subsystem 22 and saw blade.The detection subsystem includes a drive portion 100 coupled to theblade (represented by capacitor 102). Drive portion 100 is configured tocouple a drive signal onto the blade. A sense portion 104 of thedetection system is also coupled to the blade to monitor the signal onthe blade. As shown in FIG. 4, the drive signal (represented by solidline 106) typically has a voltage amplitude that varies with time suchas a sine wave, square wave, delta function, pulse, etc. The sensedsignal coupled to the blade (represented by dash line 108) essentiallymirrors the drive signal except that the amplitude of the sensed signalV_(S) is less than the amplitude of the drive signal V_(D). In addition,any resistance in the cabling between the drive/sense portions and theblade may cause a small charge/discharge delay on the blade. However, itwill be appreciated by those of skill in the art that the resistance, ifany, will typically be in the milli-ohm range so that the RC-timeconstant of the cabling/blade assembly will be on the order of a fewpicoseconds or less. (It should be noted that in FIGS. 4–8, sensedsignal 108 is shown with a greatly exaggerated charge/discharge delaydue to the RC-time constant of the cabling/blade assembly.) When aperson contacts the blade, the impedance of the person's body, indicatedat 110 in FIG. 3, is coupled to the blade. The human body impedance canbe modeled by a resistor 112 in series with a capacitor 114. Thus, aportion of the charge on blade 102 is transferred through resistor 112to capacitor 114, thereby decreasing the amplitude of the sensed voltageV_(S) to a lower level as indicated at V_(SC) in FIG. 5. The exemplarydetection subsystem described in the above-identified references detectsthis decrease in the sensed voltage amplitude and sends a signal to thecontrol subsystem which triggers the reaction subsystem. It will beappreciated that the RC-time constant of resistor 112 and capacitor 114will determine how fast a portion of the charge on capacitor 102 istransferred to capacitor 114. Typical values for the human bodycapacitance are 50–300 pF, while typical values for the human bodyresistance are approximately 1 k-ohm. As a result typical values for theRC-time constant of resistor 112 and capacitor 114 will be 50–300nanoseconds. As described in the references incorporated above, thedrive signal typically has a frequency of approximately 100–500 kHz,giving a signal period of 2–10 μsec. Thus, the RC-time constant of aperson's body is substantially invisible to the exemplary detectionsubsystems described in the above-incorporated references.

As also described in the above-identified references, other materialsmay cause a change in the sensed signal when placed in contact with theblade. For example, when very green wood is being cut by the blade, therelatively high dielectric constant of the wood may also cause theapparent capacitance of the blade to increase as the air dielectricaround the blade is replace by green wood dielectric. In other words,the capacitance of capacitor 102 in FIG. 3 is increased, therebyreducing the amplitude of sensed voltage V_(S) for a given drive voltageV_(D). In some instances, the sensed voltage V_(S) may be decreased tothe level V_(SC) as shown in FIG. 5, thereby causing control subsystem26 to trigger reaction subsystem 24. The change in apparentblade-capacitance experienced when cutting green wood accumulates (i.e.,increases to a maximum amount) over tens, hundreds or even thousands ofmilliseconds as more and more of the green wood is moved into contactwith the blade. However, the RC-time constant of the cabling/bladeassembly will remain very low.

In view of the effect of high-dielectric materials such as green wood onthe sensed voltage V_(S), some of the exemplary embodiments of detectionsubsystem 22 described in the above-mentioned references are configuredto distinguish contact between the blade and a person from contactbetween the blade and green wood to prevent erroneously triggering thereaction subsystem when cutting green wood. These detection subsystemstypically identify contact between a person and the blade based on apredetermined decrease in the sensed voltage which occurs over severalmicroseconds (V_(D) has a frequency of a few hundred kHz). As discussedabove, this time frame is much larger than the RC-time constantassociated with the human body impedance. Therefore, the person's bodyis fully charged and discharged during a small fraction of each cycle ofthe signal. In contrast, the change in apparent blade-capacitance due tocontact with green wood changes only slightly over a time frame ofseveral microseconds. Therefore, while cutting green wood can ultimatelycause a comparable decrease in the amplitude of sensed voltage V_(S),the decrease occurs over many cycles of the signal. In other words,these detection subsystems distinguish a human body from green woodbased on the rate at which the apparent capacitance of the bladechanges. If the apparent capacitance of the blade decreases to apredetermined threshold within several microseconds or tens ofmicroseconds, then detection subsystem 22 recognizes the decrease as ahuman body contact. However, if the apparent capacitance of the bladedecreases to the predetermined threshold within several hundred orthousand microseconds, then detection subsystem 22 does not recognizethe decrease as a human body contact.

Alternatively, detection subsystem 22 may be configured in any of avariety of other ways to distinguish contact between the blade and aperson from contact with high-dielectric materials such as green wood.For example, the detection subsystem may be configured to detect thecharging and/or discharging of the person's body (i.e., capacitor 102)that occurs separately from charging and discharging of the blade. Sincegreen wood merely increases the capacitance of the blade rather thanadding an additional capacitor to the detection circuit, no separatecharging or discharging occurs when green wood contacts the blade. Inother words, detection subsystem 22 may be configured to a change in theapparent capacitance of the blade, but also a change in the apparentfrequency response of the blade.

Thus, in one exemplary embodiment of detection subsystem 22, driveportion 100 is configured to drive a signal onto the blade having afrequency and/or shape adapted to output a signal to sense portion 104that indicates the charging and/or discharging of the person's body. Forexample, drive signal V_(D) may be a signal having a period comparableto the RC-time constant of the human body impedance. It will beappreciated that the drive signal may be any type of alternating signalsuch as a sine wave, square wave, delta signal, etc., or may berepeating pulses (either periodic or non-periodic) having rise and/orfall times much shorter than the RC-time constant of the human bodyimpedance. Additionally, the frequency of the drive signal may bevaried, stepped or swept over a range of frequencies to emphasize thecharge/discharge delay that is coupled to the blade when a human body isplaced into contact with the blade. The detection subsystem may beconfigured to analyze the sensed signal at each frequency to distinguishhuman body contact from contact with a high-dielectric material.Alternatively, the drive signal may have several frequency components(e.g., a 50 MHz component, a 10 MHz component, a 1 MHz component, and a500 kHz component). In such case, the detection subsystem may beconfigured to filter or otherwise separate out each component from thesensed signal to determine the frequency response of the blade circuit.Optionally, multiple sense portions 104 may be employed to analyzeand/or sense each frequency component.

FIG. 6 illustrates an exemplary drive signal 106 including a pulse width(T) on the order of approximately one RC-time constant of the human bodyimpedance. When the blade is in contact with a person's body, sensedsignal 108 initially rises to a level V_(S) corresponding to thecapacitance of the blade alone. However, as a portion of the charge onthe blade discharges into the person's body, the sensed signal decreasestoward V_(SC) corresponding to the apparent capacitance of the bladewhen coupled to the person's body. In contrast, the voltage of sensedsignal 108 does not vary during the period T when the blade contacts ahigh-dielectric material because: 1) the RC-time constant of thecabling/blade assembly is much less than T; and 2) there is no secondaryRC circuit to charge.

Similarly, FIG. 7 illustrates an exemplary drive signal 106 including apulse width (T) on the order of approximately five RC-time constants ofthe human body impedance. When the blade is in contact with a person'sbody, sensed signal 108 initially rises to V_(S), and then decreases toV_(SC) well within period T. In FIG. 8, drive signal 106 has a pulsewidth on the order of 1/10^(th) of the RC-time constant of the humanbody impedance. Consequently, sensed signal 108 does not dischargetoward V_(SC) during time period T even when a person's body is incontact with the blade.

It will be appreciated by those of skill in the art that detectionsubsystem 22 may be configured in any of a variety of ways todistinguish contact between a blade and a person from contact with othermaterials based on changes in the apparent frequency response of theblade. For example, the drive signal may be configured with multiplefrequencies, one or more having periods approximately equal to orgreater than the RC-time constant of a person's body, and one or morefrequencies having periods approximately equal to or less than theRC-time constant of a person's body. In such case, a drop in the sensedsignal voltage level at low frequencies but not at high frequencies mayindicate contact with a human body rather than other materials.Alternatively or additionally, the detection subsystem may be configuredto utilize a drive signal with a single frequency (or rise/fall time)comparable to the maximum frequency response of the blade when contactedby a human body. In which case, a decrease in the voltage level of thesensed signal from an initially high level during the period of thepulse (e.g., as shown in FIGS. 6 and 7) may indicate contact with ahuman body rather than other materials. It will be understood that whilethe examples used herein employ positive voltage pulses, the detectionsubsystem may additionally or alternatively be configured to employnegative voltage pulses to detect contact.

The exemplary detection subsystem described above may also be used todistinguish contact between the blade and a person from contact withconductive materials such as aluminum which may electrically ground theblade to other portions of machine 10. Thus a sensed signal having azero voltage would not indicate contact between the blade and a person.Since the detection subsystem may be unable to detect contact betweenthe blade and a person when the blade is grounded, it may be desirableto configure control subsystem 26 to turn off power to machine 10 if thesense signal is grounded (unless a bypass control is provided asdescribed in the above-incorporated references).

It will be appreciated that the frequency characteristics of the drivesignal employed by detection subsystem 22 to distinguish contact betweenthe blade and a person from contact between the blade and othermaterials may vary depending on the type of other materials likely tocome into contact with the blade. For example, wood products includinggreen, wet and/or pressure treated materials are typically the workpiecematerials most likely to come into contact with the cutting tools ofwoodworking machines. Alternatively, many similar machines are used forcutting other building or manufacturing materials (e.g., plastics,foams, ceramics, etc.), food products (e.g., meats, etc.), textiles,paper, etc. Therefore, depending on the particular application, it maybe desirable to analyze the frequency response of the workpiecematerials (or other materials) likely to contact the blade relative tothe frequency response of the human body to determine the optimalfrequency parameters for use by the detection subsystem.

An exemplary method for conducting such an analysis is illustrated inFIG. 9 and indicated generally at 116. The method includes measuring thefrequency response of the cutting tool when in contact with a person, asindicated at 118, and measuring the frequency response of the cuttingtool in contact with one or more other materials, as indicated at 120.For example, FIG. 10 illustrates a sample frequency response graphcorresponding to an exemplary cutting tool in contact with a person,while FIG. 11 illustrates a sample frequency response graphcorresponding to the same cutting tool in contact with another material.Method 116 continues with comparing the measured frequency responses todetermine one or more optimal frequencies at which to detect contact, asindicated at 122. Detection subsystem 22 may then be configured to sensefor contact between the blade and a person at the determinedfrequencies, indicated at 124. For example, in the exemplary embodimentsillustrated in FIGS. 10 and 11, detection system 22 might be configuredto sense for contact at one or more frequencies from f₁ to f₂, as wellat f₀ or less. In such case, attenuation of the sensed signal at f₀ butnot at f₂ would indicate contact between the blade and a person rathercontact between the blade and a material having the frequency responseshown in FIG. 11.

In an alternative embodiment, detection subsystem 22 may be configuredto automatically “tune” to the frequency response of a particularperson. For example, FIG. 12 illustrates an exemplary detectionsubsystem which includes a test electrode 126 connected to both driveportion 100 and sense portion 104. When a person's body is placed intocontact with test electrode 126, the drive signal charges and dischargesthe person's body. The detection subsystem analyzes the sensed signal todetermine the RC-time constant of the person's body. The detectionsubsystem then adjusts the frequency characteristics of the drive signalcorrespondingly so that only materials having a substantially similarfrequency response will be detected as a dangerous condition (i.e.,contact between the blade and a person). The test electrode may be aseparate, dedicated structure or may be built into any suitable portionof machine 10 which a user typically touches such as control buttons,knobs, handles, cranks, fences, etc. Control subsystem 26 may beconfigured to require a user to contact test electrode 126 prior toenabling operation of machine 10.

As described herein, safety system 18 includes a detection subsystemadapted to detect contact between a person and the cutting tools ofvarious types of woodworking machines. The detection subsystem isadapted to detect when contact occurs between the cutting tool and ahuman body, while distinguishing contact between the cutting tool andother materials which may change the electrical characteristics of thecutting tool. While several exemplary embodiments of safety system 18and detection subsystem 22 are described above, the particularembodiments that have been described serve to illustrate that manydifferent modifications and alterations are possible within the scope ofthe invention. The particular electrical implementation of detectionsubsystem 22 may utilize any of a variety of different electroniccomponents and configurations which are known to those of skill in theart.

It will be appreciated that safety system 18 and detection subsystem 22may be adapted for use on a variety of different woodworking machines.Several examples of such woodworking machines, as well as furtherdetailed descriptions of alternative safety systems may be found in thereferences incorporated above, as well as in the following references,the disclosures of which are herein incorporated by reference: PCTPatent Application Ser. No. PCT/US00/26812, filed Sep. 29, 2000; U.S.patent application Ser. No. 09/955,418, filed Sep. 17, 2001; U.S. patentapplication Ser. No. 09/929,221, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,226, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,227, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,234, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,235, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,236, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,237, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,238, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,240, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,241, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,242, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,244, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,425, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/929,426, filed Aug. 13, 2001; U.S. patentapplication Ser. No. 09/676,190, filed Sep. 29, 2000; U.S. ProvisionalPatent Application Ser. No. 60/312,141, filed Aug. 13, 2001; U.S.Provisional Patent Application Ser. No. 60/324,729, filed Sep. 24, 2001;U.S. Provisional Patent Application Ser. No. 60/323,975, filed Sep. 21,2001; U.S. Provisional Patent Application Ser. No. 60/308,492, filedJul. 27, 2001; U.S. Provisional Patent Application Ser. No. 60/307,756,filed Jul. 25, 2001; U.S. Provisional Patent Application Ser. No.60/306,202, filed Jul. 18, 2001; U.S. Provisional Patent ApplicationSer. No. 60/302,916, filed Jul. 3, 2001; U.S. Provisional PatentApplication Ser. No. 60/292,100, filed May 17, 2001; U.S. ProvisionalPatent Application Ser. No. 60/292,081, filed May 17, 2001; U.S.Provisional Patent Application Ser. No. 60/279,313, filed Mar. 27, 2001;U.S. Provisional Patent Application Ser. No. 60/275,595, filed Mar. 13,2001; U.S. Provisional Patent Application Ser. No. 60/275,594, filedMar. 13, 2001; U.S. Provisional Patent Application Ser. No. 60/273,902,filed Mar. 6, 2001; U.S. Provisional Patent Application Ser. No.60/273,178, filed Mar. 2, 2001; U.S. Provisional Patent Application Ser.No. 60/273,177, filed Mar. 2, 2001; U.S. Provisional Patent ApplicationSer. No. 60/270,942, filed Feb. 22, 2001; U.S. Provisional PatentApplication Ser. No. 60/270,941, filed Feb. 22, 2001; U.S. ProvisionalPatent Application Ser. No. 60/233,459, filed Sep. 18, 2000; U.S.Provisional Patent Application Ser. No. 60/225,210, filed Aug. 14, 2000;U.S. Provisional Patent Application Ser. No. 60/225,058, filed Aug. 14,2000; U.S. Provisional Patent Application Ser. No. 60/225,057, filedAug. 14, 2000; U.S. Provisional Patent application Ser. No. 60/182,866,filed Feb. 16, 2000; U.S. Provisional Patent Application Ser. No.60/157,340, filed Oct. 1, 1999; and U.S. Pat. No. 4,267,914, issued May19, 1981 to Saar.

It is believed that the disclosure set forth above encompasses multipledistinct inventions with independent utility. While each of theseinventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the inventions includes all novel and non-obviouscombinations and sub-combinations of the various elements, features,functions and/or properties disclosed herein. No single feature,function, element or property of the disclosed embodiments is essentialto all of the disclosed inventions. Similarly, where the claims recite“a” or “a first” element or the equivalent thereof, such claims shouldbe understood to include incorporation of one or more such elements,neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and sub-combinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and sub-combinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

1. A method of detecting contact between a human body material and aspecified portion of a woodworking machine, and of distinguishing thatcontact from contact between at least one other material and themachine, the method comprising: imparting an electrical signal on thespecified portion of the machine, where the electrical signal includesat least two different frequency components; measuring a characteristicof the electrical signal as a function of frequency when the specifiedportion is in contact with the human body material to obtain a firstfrequency response; measuring the characteristic of the electricalsignal as a function of frequency when the specified portion is incontact with the other material to obtain a second frequency response;comparing the first and second frequency responses; and settingfrequency measurement parameters to distinguish human body materialcontact from contact by the other material.
 2. The method of claim 1,where the machine is a power saw, where the specified portion is ablade, and where the method is employed to mitigate injury from contactbetween the human body material and the blade.
 3. The method of claim 1,where the electrical signal imparted to the specified portion of themachine has a period comparable to the RC-time constant of a human body.4. A method of detecting contact between a human body material and aspecified portion of a woodworking machine, and of distinguishing thatcontact from contact between at least one other material and themachine, the method comprising: imparting an electrical signal on thespecified portion of the machine, where the electrical signal includesat least two different frequency components; where the frequencycomponents include a first frequency component having a period equal toor greater than the RC-time constant of a nominal human body and asecond frequency component having a period less than the period of thefirst frequency component; measuring a characteristic of the electricalsignal as a function of frequency when the specified portion is incontact with the human body material to obtain a first frequencyresponse; measuring the characteristic of the electrical signal as afunction of frequency when the specified portion is in contact with theother material to obtain a second frequency response; comparing thefirst and second frequency responses; and setting frequency measurementparameters to distinguish human body contact from contact by the othermaterial.